The present invention rotates to an improved technique for the hydrodealkylation of a hydrodealkylatable hydrocarbon, particularly under conditions of low sulfur, which minimizes carburization thus preventing premature plant shut-downs.
The hydrodealkylation of hydrodealkylatable hydrocarbons such as alkyl aromatics has been practiced for many years. The principal processes involve the conversion of toluene and like alkyl-substituted benzenes to benzene and various byproducts. Such processes are either catalytic or non-catalytic in nature. The catalytic processes employ one or more catalysis that promote the conversion of the alkyl aromatic compounds to benzene and the remaining alkyl. The non-catalytic processes typically employ heat and pressure to promote the conversion of the alkyl aromatic compounds to benzene and the remaining alkyl.
Some conventional catalytic hydrodealkylation processes employ Group VIII metals such as Rh and Pt supported on an alumina support. For example, Kovach et al., in U.S. Pat. No. 3,700,745, describes a hydrodealkylation process which includes contacting an alkyl aromatic hydrocarbon with a catalyst including an active Group VIII metal, such as, platinum, rhodium, palladium, ruthenium and nickel. Other catalytic hydrodealkylation processes employ chromia type catalysts deposited on an alumina support. For example, Daly et al., in U.S. Pat. No. 4,451,687, discloses a catalyst for the hydrodealkylation of alkylaromatic compounds containing chromia on an alumina support. Other catalytic hydrodealkylation processes employ variations of the above catalysts or even completely different catalysts. See, for example, U.S. Pat. Nos. 3,686,340, 3,966,833, 4,189,613, 4,191,632, 4,463,206 and 5,053,574.
The catalytic processes, however, are not always suitable for the commercial conversion of alkyl aromatic compounds to benzene and the remaining alkyl. In particular, the activity, selectivity and conversion rate of such catalysts are not always suitable for large scale hydrodealkylation at the temperatures and pressures suitably employed. If the reaction temperature or pressure is increased aide reactions such as hydrocracking of the aromatic ring is promoted.
Furthermore, some catalysts tend to deactivate with use, presumably due to coke formation on the catalyst surface. In this regard, it is believed that active sites promote polymerization of either hydrogenolysis products or aromatic hydrocarbons resulting in hydrocarbon condensation on the catalyst surface. Under the conditions of the process, these condensed species are dehydrogenated forming coke. The result of these reactions is a reduction in activity of the catalyst since the coke is strongly adsorbed onto the sites which promote dealkylation. In other words, this coke or carbon build-up either blocks or poisons the active catalyst sites casing deactivation. See U.S. Pat. No. 4,451,687.
Additionally, some catalysts tend to deactivate with use, due to the presence of sulfur and in particular thiophene sulfur in the process feed. Thus, catalysts such as certain noble metal catalysts deactivate over time due to the presence of sulfur in the feed. These catalysts must be replaced or regenerated when sulfur reduces the activation to an extent low enough to prevent suitable conversion of the feed.
In view of the disadvantages associated with utilizing catalytic hydrodealkylation processes, non-catalytic hydrodealkylation processes have been developed. Mainly, such processes employ the use of heat and pressure to convert alkylaromatic compounds to benzene and the disassociated alkyl compounds.
Button et al., in U.S. Pat. No. 3,607,960, and Loboda, in U.S. Pat. No. 4,058,452, disclose processes for the thermal hydrodealkylation of an alkyl aromatic, such as toluene, to produce benzene. Both processes include subjecting a gaseous mixture of at least one alkyl aromatic compound and hydrogen in a reaction zone to a reaction temperature in the range of about 1000xc2x0 to 1800xc2x0 F. and removing benzene from the effluent. Other patents which disclose the thermal dealkylation of hydrodealkylatable hydrocarbons include U.S. Pat. Nos. 2,929,775, 3,160,671, and 3,284,526.
Thermal hydrodealkylation processes ameliorate the disadvantages associated with the above-mentioned catalytic hydrodealkylation processes in that they do not employ the use of catalysts which are susceptible to deactivation. However, due to the use of high temperatures end pressures which are required for the conversion of alkyl aromatic compounds in the absence of a suitable catalyst, such processes have their own inherent problems.
With conventional hydrodealkylation techniques, added sulfur effectively inhibits carburization. Somehow, the sulfur interferes with the carburization reaction. But with low sulfur systems or with sulfur outages, this inherent protection no longer exists especially when the system is exposed to high temperatures such as in thermal hydrodealkylation.
The problems associated with carburization include coking, carburization of system metallurgy, and metal dusting. The embrittlement of the steel walls by carburization leads to xe2x80x9cmetal-dustingxe2x80x9d, i.e., a release of catalytically active particles and metal droplets of metal due to an erosion of the metal. The excessive xe2x80x9cmetal-dustingxe2x80x9d adds active metal particulates to the system, which particulates provide additional sites for coke formation in the system.
Coking is generally not a problem which must be addressed in hydrodealkylation processes, but this significant source of calm formation due to the absence of sulfur in reactor feedstreams excessively aggravates the problem. In fact, active metal particulates in coke particles metastasize more generally throughout the system. That is, the active metal particulates actually induce coke formation on themselves and anywhere that the particles accumulate in the system resulting in coke plugs and hot regions of exothermic reactions. As a result, a premature coke-plugging of the reactor system occurs which can lead to a premature shut-down of the system.
One solution to the problem associated with carburization, embrittlement, and metal-dusting is to add sulfur to the feed to thereby effectively inhibit carburization. However, the addition of sulfur increases production cost and process complexity. Moreover, sulfur usage has inherent environmental and safety hazards which are preferably avoided. Moreover, at high temperatures coking and carburization will still occur even when sulfur is added to the feed.
Consequently, there remains a need in the art for improved processes for the hydrodealkylation of hydrodealkylatable compounds with reduced carburization, especially in the absence of, or at low levels of, sulfur. Such a method would include means for inhibiting the undesirable catalytic activity which causes carburization of system metal.
Accordingly, one object of the invention is to provide the technical background necessary for solutions to the problems associated with coking, carburization, and metal-dusting. In particular, the discovery of the mechanisms involved with carburization and metal-dusting which lead to premature coke-plugging allows those skilled in the art to formulate solutions to the problems.
Another object of this invention is to provide a method for inhibiting carburization and metal-dusting in a process for thermally dealkylating a hydrodealkylatable hydrocarbon. The process includes pretreating surfaces exposed to the dealkylation atmosphere to form a protective layer which is more resistant to embrittlement, carburization and metal-dusting than the materials conventionally used to manufacture the reactor system.
The use of the process of the present invention allows for the dealkylation of hydrodealkylatable hydrocarbons especially in the absence of sulfur. Thus, an advantage of the method for inhibiting carburization includes the lack of a need for the addition of sulfur to hydrocarbon feeds and any recycle streams. In addition, hydrocarbon foods having a low sulfur content may be used.
More preferably, the invention relates to a method for thermally hydrodealkylating hydrocarbons comprising contacting the hydrocarbons with hydrogen in a reactor system having as resistance to carburization and metal-dusting, which is an improvement over conventional steel reactor systems under conditions of low sulfur.
Another aspect of the invention includes providing a reactor system which prevents embrittlement. Preventing embrittlement significantly reduces metal-dusting and coking in the reactor system, and permits operation for longer periods of time. Furthermore, higher temperatures may be used during hydrodealkylation, especially in thermal hydrodealkylation processes, allowing for increased production.
Even another preferred embodiment relates to the discovery that simply providing a protective plating, cladding or other coating such as a paint, to a reactor system will not necessarily be sufficient to completely address the aforementioned problems. Such a protective layer must be of sufficient thickness to provide a complete, uninterrupted coating of the underlying base metal, and it must remain complete over time. Even minor imperfections, pinholes or other flaws in the protective layer an provide destructive carburization sites sufficient to shutdown operation.
An effective protective layer must resist deleterious chemical alteration, as well as peeling and/or splitting. Additionally, it has been found that any protective layer must be applied to a thickness sufficient to completely cover the surface to be protected, and must maintain its integrity through operation. As such, the protective layer must be sufficiently abrasion resistant during catalyst loading, start-up and operation.
According to this preferred embodiment them is used an intermediate bonding layer which anchors the protective layer to the steel substrate to be protected. In this regard, the reactor system comprises a steel portion having provided thereon a protective layer to isolate the steel portion from hydrocarbons, preferably a stannide layer, applied to a thickness effective for completely isolating the steel portion from the hydrocarbon environment, while avoiding any substantial liquid metal embrittlement. The protective layer is anchored to the steel substrate through an intermediate carbide-rich (relative to the underlying steel), bonding layer; in the case of a stainless steel substrate, an intermediate carbide-rich, nickel-depleted (relative to the underlying steel) bonding layer.
In the case of a stannide outer protective layer and a stainless steel substrate, the stannide layer is nickel-enriched and comprises carbide inclusions, while the intermediate carbide-rich, nickel-depleted bonding layer comprises stannide inclusions. Preferably the carbide inclusions are continuous extensions or projections of the bonding layer as they extend, substantially without interruption, from the intermediate carbide-rich, nickel-depleted bonding layer into the stannide phase, and the stannide inclusions are likewise continuous extending from the stannide layer into the intermediate carbide-rich, nickel-depleted bonding layer. The aforementioned presence of carbide inclusions in the stannide layer, and stannide inclusions in the intermediate carbide-rich, nickel-depleted bonding layer, provide improved anchoring of the protective layer thereby increasing abrasion resistance. The interface between the intermediate carbide-rich, bonding layer and the nickel-enriched stannide layer is irregular, but is otherwise substantially without interruption.
Although there is a need to ensure a complete coating of the underlying base metal to be protected, applying excessive amounts or thicknesses of the material used to form the protective layer must also be avoided. If the layer is too thick, for example, where the alloying materials of a prior have locally pooled prior to curing, liquid metal embrittlement can occur. The problem of liquid metal embrittlement is essentially one of eating through the metal with alloying materials (such as tin or germanium) which are extremely corrosive to steel under reducing conditions to the point where, again, the metallurgy fails.
Additionally, it has been found that certain preferred coatings are sulfur-tolerant, for example, the tin-based protective layers can tolerate up to 200 ppm sulfur. The protective layers eliminate the need to presulfide the metallurgy, reduce sulfide corrosion and improve product values and waste disposal due to induced levels of sulfur. Chromium-, Sb- and Ge-based protective layers can tolerate even higher sulfur levels, up to 5 or more wt. %. Most preferably, the layers can tolerate the respective amounts of sulfur for a period of at least 200, preferably at least 400, and most preferably 600 hours without degrading to an extent that carburization will occur, resulting in shut-down of the system due to excessive coking.
With the foregoing, as well as other objects, advantages, features and aspects of the disclosure that will become hereinafter apparent, the nature of the disclosure may be more clearly understood by reference to the detailed description and the appended claims.